The present invention relates to a method for densifying a porous refractory substrate so as to provide a high strength surface having increased flatwise tensile strength.
Portions of the space shuttle orbiter, which is subject to temperatures ranging from -250.degree. F. to 2300.degree. F., utilize tiles formed of substantially chemically pure silica fibers which are rigidized with a high purity silica binder. The tiles, known as LI-900 and LI-2200, are manufactured by Lockheed Missles and Space Company, Inc., have a density of 9 pounds and 22 pounds per cubic foot respectively, relatively low strength, extremely high temperature resistance, and an extremely low coefficient of thermal expansion as compared to metals. As disclosed in U.S. Pat. No. 4,124,732, incorporated herein by reference for all purposes, the tiles are secured to the space shuttle orbiter by means of a strain isolator pad which is interposed between the tiles and the surface of the structure to be protected. It has been found that the tiles currently used having a density of 9 pounds per cubic foot are subject to interfacial tile-isolator pad failure at about 11 psi of flatwise tensile stress. Interfacial bond failure is caused by uneven tension exerted by the strain isolator pad. Accordingly, if the surface of the tile at the tile-strain isolator pad interface were of higher strength and the bond between the tile and the isolator pad increased, any failure caused by flatwise tensile stress could be shifted into the body of the tile thus achieving the highest possible flatwise tensile strength in the interfacial bond.
U.S. Pat. No. 4,124,732 teaches a method of securing the tiles to a surface of a structure sought to be protected. U.S. Pat. No. 2,791,262 discloses a process for improving the physical characteristics of sheet form which includes the steps of forming a mixture of mica pulp and hydrolyzed ethyl orthosilicate and forming a sheet from the above mixture by curing. U.S. Pat. No. 3,702,279 discloses a method for forming low density insulation comprising the steps of molding a slurry of inorganic fibers, impregnating the molded fiberous mass with a binder of an inorganic sol, gelatinizing the sol, curing the gelatinized binder, drying the composite and firing the composite to sinter the binder. The patent discloses the preparation of the silica sol from ethyl silicate, ethyl alcohol and HCL. None of the references discussed above, which are believed by applicants to be the most pertinent, teach a method of locally densifying a porous refractory or ceramic material whereby the bond between such material and the surface to which it is attached may be increased to the point such that any failure caused by flatwise tensile pressure is shifted from the attached surface area into the body of the material.